This command should install
pip install pymbolic
You may need to run this with sudo. If you don’t already have pip, run this beforehand:
curl -O https://raw.github.com/pypa/pip/master/contrib/get-pip.py python get-pip.py
For a more manual installation, download the source, unpack it, and say:
python setup.py install
Why pymbolic when there’s already sympy?¶
(This is extracted from an email I (Andreas) sent to Aaron Meurer and Anthony Scopatz.)
So why not use
sympy as an AST for DSLs and code generation? It’s a good
question. As you read the points I make below, please bear in mind that I’m not
saying this to ‘attack’ sympy or to diminish the achievement that it is. Very
much on the contrary–as I said above, sympy does a fantastic job being a
computer algebra. I just don’t think it’s as much in its element as an IR for
code generation. Personally, I think that’s perfectly fine–IMO, the tradeoffs
are different for IRs and efficient computer algebra. In a sense, pymbolic
competes much harder with Python’s ast module for being a usable program
representation than with Sympy for being a CAS.
At any rate, to answer your question, here goes:
First, specifically because sympy is smart about its input, and will rewrite it behind your back. pymbolic is intended to be a dumb and static expression tree, and it will leave its input alone unless you explicitly tell it not to. In terms of floating point math or around custom node types that may or may not obey the same rules as scalars, I feel like ‘leave it alone’ is a safer default.
The very moment code generation starts taking more than a second or so, you’ll want to implement a caching mechanism, likely using Pickle.
Extensibility of transformation constructs: sympy’s built-in traversal behaviors (e.g. taking derivatives, conversion to string, code generation) aren’t particularly easy to extend. It’s important to understand what I’m talking about here: I would like to be able to make something that, say, is like taking a derivative (or evaluating, or…), but behaves just a bit differently for a few node types. This is a need that I’ve found to be very common in code generation. In (my understanding of) sympy, these behaviors are attached to method names, so the only way I could conceivably obtain a tweaked “diff” would be to temporarily monkeypatch “diff” for my node type, which is kind of a nonstarter. (unless I’m missing something)
Pymbolic’s “mapper” mechanism does somewhat better here–you simply inherit from the base behavior, implement/override a few methods, and you’re done.
This part is a bit of a red herring though, since this can be implemented for sympy (and, in fact, I have). Also, I noticed that sympy’s codegen module implements something similar (e.g. here). The remaining issue is that most of sympy’s behaviors aren’t available to extend in this style.
Representation of code-like constructs, such as:
Bit shifts and other bitwise ops:
Distinguishing floor-div and true-div
I should also mention that pymbolic, aside from maintenance and bug fixes, is effectively ‘finished’. It’s pretty tiny, it’s not ambitious, and it’s not going to change much going forward. And that is precisely what I want from a package that provides the core data structure for something complicated and compiler-ish that I’m building on top.
pymbolic is licensed to you under the MIT/X Consortium license:
Copyright (c) 2008-13 Andreas Klöckner
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.